U.S. Pat. No. 4,217,132 to Burge et al, incorporated herein by reference, describes a method and apparatus for in-flight combustion of carbonaceous fuel in a manner and under conditions such that substantially all slag is removed from the gaseous products of combustion. In the system described therein, a carbonaceous fuel, such as particulate coal, fluidized by a carrier fluid is injected into a swirling combustion zone into which oxidizer, typically air, is introduced tangentially to aid in maintaining swirling conditions. Combustion occurs with formation of slag. Slag centrifugally accumulates along the interior walls of the combustion chamber and flows to a baffle where the slag is collected and removed to a slag-disposal subsystem. Slag serves as an insulating layer to protect the materials forming the walls of the combustor. The slag primarily consists of noncombustible minerals contained in the coal and is essentially carbon-free. The gaseous product of the slagging combustor may, for instance, be introduced with additional oxygen to a secondary combustion system for the generation of thermal energy.
Most coal contains substantial amounts of sulfur some of which is organically bound with the carbon, and some present in the form of metal sulfides and sulfates Societal considerations, including current clean-air regulations, dictate reduction of sulfur oxide emissions to concentrations lower than heretofore readily achievable in high power density coal combustion. As disclosed at column 4, lines 51 et seq. of the patent, the problem of removal of the oxides of sulfur was addressed. There, it was stated that to control sulfur oxide emissions, treatment chemicals such as carbonates may be added to the input fuel, or introduced separately to the reaction chamber.